I think that being pedantic "Sync speed" is a more accurate term than "mechanical speed", after all most if not all of the earlier focal plane shutters were fully mechanical. I would go for "maximum synch speed", as you can of course synchronise at lower speeds.

Just MHO

I guess it's worth mentioning that some modern flash/camera pairings give sync at all speeds with focal plane shutters using something called FP mode. I guess this works by strobing the flash, but I've no real tech info on it.